Huckel MO Theory and Conjugated Systems

Question 1

Define conjugated system

Answer 1

π-bonds formed by side-on overlap of adjacent p-orbitals

-all p-orbitals must be in the same plane

Question 2

About Huckel MO theory

Answer 2

Rationalize p orbital interactions and relative energies of resulting p-orbitals

• the p-orbitals are treated separately from the sigma-framework
• # π MOs = # of contributing p-orbitals
• phase change = node
• more nodes = higher energy MO

Question 3

Define alpha

Answer 3

• the energy released when a single e- (in vacuum) is put in a p-orbital
• this is a negative term (stabilizing factor)
• called the coulomb integral

Question 4

Define ß

Answer 4

how much more stable is the system when the p-orbitals are in conjugation

• this is a negative term (stabilizing factor)
• called the resonance integral
• larger values of ß are a result of more stabilizing resonance, so more stable MOs

Question 5

Equation for resonance coefficient

Answer 5

mi = 2cos(πi/(n+1)) OR mi = 2cos(180i/(n+1)
where n is the number of conjugated carbons
i is an integer, STARTING AT 1, all positive

Question 6

MO energy equation

Answer 6

Eπi = alpha + 2ßcos(180i/(n+1))

Question 7

Allyl systems

Answer 7

allyl cation, radical and carbanion have equal resonance energy - same # of resonance structures

Question 8

p-orbital coefficients

Answer 8

-orbital coefficients reflect their contribution to individual MOs
symbol is c_ri

Question 9

Frontier orbitals

Answer 9

MOs immediately above/below alpha level are frontier orbitals:
HOMO- highest occupied molecular orbital
LUMO- lowest unoccupied molecular orbital
SOMO- singly occupied molecular orbital
NBMO - nonbonding molecular orbital (if there are an odd # of C, there will be a NBMO at alpha)

Question 10

HMOT of linear π systems

Answer 10

• conjugated system with n centres must have n orbitals
• lowest energy MO has zero nodes, second has one, etc.
• nodes are symmetrical about the centre of the molecule.

Question 11

Rules for aromatic system

Answer 11

• planar
• cyclic
• conjugated
• 4n+2 π e-

Question 12

Rules for antiaromatic system

Answer 12

• planar
• cyclic
• conjugated
• 4n π e-

Question 13

Frost circle

Answer 13

• Used to assess relative π-MO energies
• draw the ring point down, in a circle with radius 2ß
• label vertical axis energy

Question 14

Aromatic MOs energy equation

Answer 14

Eπi = alpha +2ßcos(2i(180)/N)
i = 0, ±1, ±2, ... N/2
N = number of p-orbitals in aromatic system

Question 15

Which cyclic system is open shelled?

Answer 15

antiaromatic

• unpaired e-
• extremely reactive

Question 16

What is Mobius Aromaticity

Answer 16

• systems with an odd number of nodes and 4n electrons
• only really accommodated for large ring systems
• can have a mlc that has 4n e- that is stable if it is Mobius aromatic

Question 17

Drawing Frost diagram for Mobius

Answer 17

• draw edge down (rather than point)

- circle with radius 2ß